In the use of conventional luminaire apparatus, there are many instances where lamp failure may become a problem. This has led to various proposals for providing a back-up or auxiliary source of light which can be utilized to replace a faulty lamp quickly and effectively. This is especially true in connection with cap lamp apparatus where a headpiece is attached to the cap of a user. A particular case in point is the headpiece customarily worn by a miner under ground where it is highly important from the standpoint of miner safety that the back-up light be capable of projecting reflected radiation without distortion, shadows or other objectionable lighting patterns.
Various proposals have been made in the art to provide an auxiliary or back-up light source. One approach has been to utilize two bulbs located adjacent to one another in a reflector chamber. Devices of this type are disclosed for example in U.S. Pat. Nos. 1,361,557; 1,757,888; 2,623,158; 2,794,114. Means are also combined with two bulbs in some of these devices for preventing explosive gases being ignited by an incandescent filament.
These patented devices have been found to be objectionable in operation for various reasons. It has not been possible to place the back-up light anywhere near the focal point of the reflector system. Therefore, the resulting light distribution pattern must necessarily be grossly distorted. Also with any of the various lamp arrangements described, a lamp means which is not being used will cast shadows and produce distortions of an objectionable nature.
Another approach to providing a back-up light source has been to furnish a plurality of light sources any one of which may be moved into an operative position in various classes of luminaire apparatus. U.S. Pat. No. 1,830,537 discloses a motion picture projection apparatus having a dual lamp source rotatably mounted in the frame of the projection apparatus. U.S. Pat. No. 2,054,013 discloses a flashing light signal device in which a plurality of lamps are mounted for rotary movement in a carrier body. U.S. Pat. No. 2,032,515 discloses an automobile headlight in which are supported a plurality of lamp elements mounted for reciprocating movement in a spring loaded holder and rotatable to selectively engage through an orifice in a headlight reflector.